1. Field of the Invention
This invention relates to a method for producing an optical disc, such as a digital audio disc, that is a compact disc, or a magneto-optical disc. More particularly, it relates to an improved method for producing an optical disc by a photopolymerization method, according to which a pattern of projections and recesses is transferred to a glass base plate by photopolymerization of a synthetic resin curable with ultraviolet rays.
2. Related Art Statement
For producing an optical disc, such as a digital audio disc, that is a so-called compact disc, or a magneto-optical disc, there is proposed, as a technique for transferring the pattern of alternate projections and recesses, inverted from pits and grooves, onto the base plate, a photopolymerization method, according to which a stamper having such pattern thereon is brought into tight contact with a liquid resin curable with ultraviolet rays and irradiated by ultraviolet rays for photopolymerizing the resin.
More specifically, the photopolymerization method comprises the steps of coating a liquid resin curable with ultraviolet rays on the surface of a light transmitting base plate for a disc by, for example, screen printing, bringing a stamper having a pattern of fine projections and recesses on its surface into intimate contact with the resin by a roller irradiating the base plate side of the resulting base plate-stamper assembly with ultraviolet rays for photopolymerizing and thereby curing the resin, and peeling the base plate and the cured resin layer from the stamper for transferring the fine pattern of alternate projections and recesses, which are the inversions of the pits and grooves, from the stamper surface onto the base plate surface.
Since the resin curable with ultraviolet rays is brought in the liquid state into tight contact with the stamper, duplication of the pattern may be achieved with higher accuracy, while the risk of stamper deterioration may be eliminated almost completely. Above all, the base plate with excellent dimensional stability and thermal resistance and favorable birefringence may be obtained with the use of the glass base plate.
Meanwhile, for transferring the pattern of alternate projections and recesses on the glass base plate by the photopolymerization method, a problem is raised in connection with the bonding properties between the glass base plate and the resin curable with ultraviolet rays.
With unsatisfactory bonding properties between the glass base plate and the resin curable with ultraviolet rays, there is the risk that, when the base plate is peeled off from the stamper, the resin layer to which the pattern has been transferred from the stamper, remains affixed to the stamper, or defects or drosses are produced in the cured resin on the glass base plate.
In view of the foregoing, attempts have been made for processing the surface of the glass base plate by a coupling agent in advance for procuring adhesive or bonding properties between the glass base plate and the resin, as disclosed in Japanese Patent Publication No. JP-A-62-71038.
The coupling agent includes both the functional group capable of bonding with an inorganic material, such as glass, and the functional group capable of bonding with the resin curable with ultraviolet rays, and thus possibly acts for briding the glass and resin for firmly bonding the glass base plate and the resin curable with ultraviolet rays to each other.
However, it has been found by our further researches that, despite surface processing by the coupling agent, peeling of the resin layer occurs frequently due to an insufficient bonding power, such that there is still ample room for improvement if the optical disc with extremely high reliability is to be produced.